1. Technical Field
The present disclosure relates to the electronics field. Particularly, this solution concerns switching direct current-direct current (dc-dc) converters.
2. Description of the Related Art
In electronics, a dc-dc converter is an electronic circuit adapted to convert a dc voltage value to another one. An important application field of dc-dc converters regards the electronic systems supplied through the power grid. Particularly, such electronic systems generally comprise a front-end circuit capable of generating a dc voltage by rectifying the (alternating) voltage provided by the power grid; however, a generic electronic system is typically formed by a plurality of sub-circuits, each needing a different supply voltage value. The presence of one or more dc-dc converters allows to locally generate said different supply voltage values starting from the one generated by the front-end circuit.
Among the various dc-dc converters presently available on the market, a well-known class thereof is represented by the so-called switching dc-dc converters. A switching dc-dc converter includes one or more switching elements (such as power MOS transistors) that are properly switched for generating a square wave starting from the dc input supply voltage.
The switching frequency of the switching elements is significantly higher than that of the alternating voltage provided by the power grid. Therefore, the transformer included in a switching converter can have a smaller size with respect to that of a transformer designed to be directly fed by the power grid. Moreover, a switching converter is characterized by high efficiency and low heat generation.
However, while operating at higher frequencies allows a considerable reduction in the size of the passive components included in a switching converter, such as the transformers and the filters, a high switching frequency entails an increase of the so-called driving losses and of the so-called switching losses. While driving losses are caused by the electrical power required for switching the switching elements, there are two different types of switching losses. A first type of switching losses is given by the simultaneous presence of current through the switching elements and voltage across their terminals during the switching thereof (“hard switching” condition). The second type of switching losses, typically called capacitive losses, is caused by the parasitic capacitance associated with each switching element, which is discharged on the resistance of the switching element itself while this is activated. Both capacitive and switching losses are proportional to the operating frequency of the switching elements. Capacitive losses are proportional to the squared switched voltage
To reduce switching losses and allow high frequency operation, resonant conversion techniques have been widely developed. These techniques provide for processing electrical power in a sinusoidal manner, and controlling the switching elements in such a way to limit the occurrence of hard switching.
Resonant converters operated from the rectified power grid voltage are typically realized using the half-bridge and the full-bridge topologies.
With reference to resonant dc-dc converters implemented according to the so-called half bridge topology (briefly referred to as half-bridge resonant converters), the switching elements include a high-side transistor and a low-side transistor connected in series between the supply circuit providing the supply voltage to be converted and a terminal providing a reference voltage, such as ground. By properly switching said two transistors it is possible to generate a square wave having a high value (assumed when the high-side transistor is activated) corresponding to the supply voltage and a low value (assumed when the low-side transistor is activated) corresponding to the ground. A small dead-time, where both transistors are off, is typically inserted as soon as each transistor turns off.
The same square wave may be generated by using two pairs of switching elements each one arranged according to the half-bridge topology, but driven in phase opposition to each other. Particularly, the high-side transistor of the first pair and the low-side transistor of the second pair are activated simultaneously; similarly, the low-side transistor of the first pair and the high-side transistor of the second pair are activated simultaneously. This topology is generally referred to as full-bridge topology. A resonant converter based on a full-bridge topology is briefly referred to as a full-bridge resonant converter.
In a resonant converter, the square wave, generated with either a half-bridge or a full-bridge topology, is applied to the primary winding of a transformer through a resonant network that includes at least a capacitor and an inductor; the secondary winding of said transformer feeds a rectifier circuit and a filter circuit for providing an output dc voltage. The value of the output dc voltage depends on the frequency of the square wave, whether it gets closer to or further from the resonance frequency of the resonant network. The duty cycle of the square wave is kept at about 50%.
Among the various known configurations of resonant network in resonant converters, the so-called inductor-inductor-capacitor (LLC) configuration is especially suited for those applications in which the value of the dc voltage to be converted is particularly high, such as the one generated through the rectification of the power grid voltage, i.e., in a condition favorable for the occurrence of high capacitive losses. The resonant network of an LLC resonant converter is formed by a series inductor-capacitor (LC) circuit connected between the switching elements and an input of the primary winding of the transformer, and a shunt inductor connected across both the inputs of the primary winding.
With an LLC resonant converter, it is possible to adjust the value of the output dc voltage over wide load and input dc voltage variations with a relatively small variation of the switching frequency. Moreover, the LLC topology allows achieving a Zero Voltage Switching (ZVS) condition—wherein the transistors forming the switching elements switch with a nearly zero drain-to-source voltage—with ease. Particularly, by properly designing the resonant network in such a way that the reactive component of its impedance is inductive for a sufficiently large switching frequency range, the current flowing into the resonant network lags the voltage square wave generated by the switching elements. Referring to the half-bridge topology for simplicity, under this condition, when the high-side transistor turns off the current is still positive (entering into the resonant network). This forces the intermediate node shared by the high-side and the low-side transistors to fall to ground so that the current flows through the body diode of the low-side transistor. When the low-side transistor is switched on after the dead-time, its drain-to-source voltage is essentially zero. Similarly, when the low-side transistor turns off, the current is still negative (coming out from the resonant network). This forces the intermediate node shared by the transistors to rise to the input voltage so that the current flows through the body diode of the high-side transistor. When the high-side transistor is switched on after the dead-time, its drain-to-source voltage is substantially zero. Thus, both the high-side and the low-side transistors are switched on in the ZVS condition.
It has to be appreciated that with an appropriate design of the transformer that couples the resonant network to the output rectifiers, the inductive components of the LLC resonant network can be “integrated” inside the transformer itself, so that no additional physical device is required for the implementation of the series and shunt inductors. In this case, the transformer is referred to as “resonant transformer”.
Resonant converters, and particularly those having the half-bridge topology, are affected by a quite serious drawback occurring during the start-up phase. Particularly, in steady state, the voltage across the terminals of the capacitor included in the resonant network comprises a dc component, corresponding to about half the supply voltage provided by the supply circuit, and an ac component that follows the course in time of the square wave. Since the capacitor blocks the dc component of such voltage, the voltage across the primary winding of the transformer exhibits the ac component only; as a consequence, in steady state, the value of the magnetic flux linking the primary winding with the secondary winding of the transformer oscillates within a symmetrical range defined by such ac component only. On the contrary, at the start-up of the converter, the capacitor is discharged; thus, when the high-side transistor switches on for the first time, the voltage seen by the primary winding is substantially equal to the input supply voltage. In the subsequent semi-period of the square wave, when the low-side transistor is switched on, the voltage seen by the primary winding is the one developed across the resonant capacitor, which is still low. Consequently, when the high-side transistor is turned on for the first time, the current flowing into the resonant network increases more rapidly than it decreases when the low-side transistor is turned on in the subsequent semi-period. When the low-side transistor is turned off again, the current is still flowing through the body diode of the low-side transistor itself; when the high-side transistor is turned on in the subsequent cycle, a reverse voltage is developed across the body diode of the low-side transistor while the latter transistor is still conducting. In this condition, the high-side transistor is switched on in a hard switching condition, with a large current flowing theretrough until the body diode of the low-side transistor is recovered. As a consequence, the high-side and the low-side transistors results to be conductive at the same time (shoot-through condition), thus short-circuiting the terminal providing the supply voltage to be converted with the terminal providing the ground voltage until the recovery of the body diode is over. In this condition, in addition to the high peak of current, which wastes a high amount of instantaneous power, the voltages across the terminals of the transistors may rapidly vary at such a rate that the parasitic silicon-controlled rectifiers (SCRs) inherent in the structure of the transistors may be triggered, thus originating a permanent shoot-through condition capable of causing the destruction of the transistors in few microseconds.
In a converter having the full-bridge topology, since there is no dc voltage across the capacitor included in the resonant network, the above mentioned start-up issue is much less severe, but not excluded.